Brexit campaign group Leave.EU has been fined a record-equalling £70,000 and its chief executive has been referred to police over its spending during the EU referendum campaign, the Electoral Commission said.

The pro-leave group, co-founded by former Ukip backer Arron Banks, failed to include £77,380 in its spending returns, taking it 10% over the limit for non-party registered groups, an investigation found.

The commission said the actual overspend figure "may well have been considerably higher", adding that the group had presented "incomplete and inaccurate" information.

We have fined https://t.co/JFd2UeRquB £70,000 for offences committed under electoral law, following our investigation into the campaigner's funding and spend during the EU referendum.

Leave.EU's chief executive, Liz Bilney, who signed the spending returns, has been referred to the Metropolitan Police, with the commission saying it had "reasonable grounds to suspect that the responsible person for Leave.EU committed criminal offences".

Mr Banks attacked the Electoral Commission (EC) and threatened to take legal action over its findings.

He said: "We view the Electoral Commission announcement as a politically-motivated attack on Brexit and the 17.4 million people who defied the establishment to vote for an independent Britain."

Leave.EU chairman Arron Bank (Jonathan Brady/PA)

He added: "The EC went big game fishing and found a few 'aged' dead sardines on the beach. So much for the big conspiracy!

"What a shambles. We will see them in court."

Speaking on BBC Radio 4's Today programme, he added that the commission was made up of "former MPs, liberal MPs, the SNP, former Labour leaders of councils, all sorts of people that all believe in Remain".

The £70,000 fine handed out by the Electoral Commission matches its previous record financial censure, handed out in March 2017 to the Conservatives for offences during the 2015 general election campaign and by-elections the previous year.

A separate probe into spending by the official Leave campaign, Vote Leave, is continuing, the EC said.

The £77,380 which was not included in the spending was fees paid to the company Better For The Country Limited, the campaign organiser.

Our investigation concluded that https://t.co/JFd2UeRquB incorrectly reported what it spent at the EU referendum. It exceeded its statutory spending limit and delivered incomplete and inaccurate spending and transaction returns.

She told Today: "The decisions were made by directors of the Electoral Commission, they were made based on robust evidence we had gathered, and indeed you can see that in the 30-page report that we published."

Ms Bassett added that there was evidence of problems with financial reporting, saying: "It is quite clear that those transaction returns contained inaccurate information about purported loans to Leave.EU and it also contains significant errors about how money was spent - money was missing and invoices were missing."